Migrations

Migrations are a convenient way for you to alter your database in a structured
and organized manner. You could edit fragments of SQL by hand but you would then
be responsible for telling other developers that they need to go and run them.
You’d also have to keep track of which changes need to be run against the
production machines next time you deploy.

Active Record tracks which migrations have already been run so all you have to
do is update your source and run rake db:migrate. Active Record will work out
which migrations should be run. It will also update your db/schema.rb file to
match the structure of your database.

Migrations also allow you to describe these transformations using Ruby. The
great thing about this is that (like most of Active Record’s functionality) it
is database independent: you don’t need to worry about the precise syntax of
CREATE TABLE any more than you worry about variations on SELECT * (you can
drop down to raw SQL for database specific features). For example you could use
SQLite3 in development, but MySQL in production.

1 Anatomy of a Migration

Before we dive into the details of a migration, here are a few examples of the
sorts of things you can do:

class CreateProducts < ActiveRecord::Migration
def up
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
def down
drop_table :products
end
end

This migration adds a table called products with a string column called name
and a text column called description. A primary key column called id will
also be added, however since this is the default we do not need to ask for this.
The timestamp columns created_at and updated_at which Active Record
populates automatically will also be added. Reversing this migration is as
simple as dropping the table.

Migrations are not limited to changing the schema. You can also use them to fix
bad data in the database or populate new fields:

This migration adds a receive_newsletter column to the users table. We want
it to default to false for new users, but existing users are considered to
have already opted in, so we use the User model to set the flag to true for
existing users.

Rails 3.1 makes migrations smarter by providing a new change method.
This method is preferred for writing constructive migrations (adding columns or
tables). The migration knows how to migrate your database and reverse it when
the migration is rolled back without the need to write a separate down method.

1.1 Migrations are Classes

A migration is a subclass of ActiveRecord::Migration that implements
two methods: up (perform the required transformations) and down (revert
them).

Active Record provides methods that perform common data definition tasks in a
database independent way (you’ll read about them in detail later):

add_column

add_index

change_column

change_table

create_table

drop_table

remove_column

remove_index

rename_column

If you need to perform tasks specific to your database (for example create a
foreign key constraint) then the
execute method allows you to execute arbitrary SQL. A migration is just a
regular Ruby class so you’re not limited to these functions. For example after
adding a column you could write code to set the value of that column for
existing records (if necessary using your models).

On databases that support transactions with statements that change the schema
(such as PostgreSQL or SQLite3), migrations are wrapped in a transaction. If the
database does not support this (for example MySQL) then when a migration fails
the parts of it that succeeded will not be rolled back. You will have to rollback
the changes that were made by hand.

1.2 What’s in a Name

Migrations are stored as files in the db/migrate directory, one for each
migration class. The name of the file is of the form
YYYYMMDDHHMMSS_create_products.rb, that is to say a UTC timestamp
identifying the migration followed by an underscore followed by the name
of the migration. The name of the migration class (CamelCased version)
should match the latter part of the file name. For example
20080906120000_create_products.rb should define class CreateProducts and
20080906120001_add_details_to_products.rb should define
AddDetailsToProducts. If you do feel the need to change the file name then you
have to update the name of the class inside or Rails will complain
about a missing class.

Internally Rails only uses the migration’s number (the timestamp) to identify
them. Prior to Rails 2.1 the migration number started at 1 and was incremented
each time a migration was generated. With multiple developers it was easy for
these to clash requiring you to rollback migrations and renumber them. With
Rails 2.1+ this is largely avoided by using the creation time of the migration
to identify them. You can revert to the old numbering scheme by adding the
following line to config/application.rb.

config.active_record.timestamped_migrations = false

The combination of timestamps and recording which migrations have been run
allows Rails to handle common situations that occur with multiple developers.

For example Alice adds migrations 20080906120000 and 20080906123000 and Bob
adds 20080906124500 and runs it. Alice finishes her changes and checks in her
migrations and Bob pulls down the latest changes. When Bob runs rake db:migrate,
Rails knows that it has not run Alice’s two migrations so it executes the up method for each migration.

Of course this is no substitution for communication within the team. For
example, if Alice’s migration removed a table that Bob’s migration assumed to
exist, then trouble would certainly strike.

1.3 Changing Migrations

Occasionally you will make a mistake when writing a migration. If you have
already run the migration then you cannot just edit the migration and run the
migration again: Rails thinks it has already run the migration and so will do
nothing when you run rake db:migrate. You must rollback the migration (for
example with rake db:rollback), edit your migration and then run rake db:migrate to run the corrected version.

In general editing existing migrations is not a good idea: you will be creating
extra work for yourself and your co-workers and cause major headaches if the
existing version of the migration has already been run on production machines.
Instead, you should write a new migration that performs the changes you require.
Editing a freshly generated migration that has not yet been committed to source
control (or, more generally, which has not been propagated beyond your
development machine) is relatively harmless.

1.4 Supported Types

Active Record supports the following database column types:

:binary

:boolean

:date

:datetime

:decimal

:float

:integer

:primary_key

:string

:text

:time

:timestamp

These will be mapped onto an appropriate underlying database type. For example,
with MySQL the type :string is mapped to VARCHAR(255). You can create
columns of types not supported by Active Record when using the non-sexy syntax,
for example

2 Creating a Migration

2.1 Creating a Model

The model and scaffold generators will create migrations appropriate for adding
a new model. This migration will already contain instructions for creating the
relevant table. If you tell Rails what columns you want, then statements for
adding these columns will also be created. For example, running

You can append as many column name/type pairs as you want. By default, the
generated migration will include t.timestamps (which creates the
updated_at and created_at columns that are automatically populated
by Active Record).

2.2 Creating a Standalone Migration

If you are creating migrations for other purposes (for example to add a column
to an existing table) then you can also use the migration generator:

If the migration name is of the form “AddXXXToYYY” or “RemoveXXXFromYYY” and is
followed by a list of column names and types then a migration containing the
appropriate add_column and remove_column statements will be created.

As always, what has been generated for you is just a starting point. You can add
or remove from it as you see fit by editing the
db/migrate/YYYYMMDDHHMMSS_add_details_to_products.rb file.

The generated migration file for destructive migrations will still be
old-style using the up and down methods. This is because Rails needs to know
the original data types defined when you made the original changes.

3 Writing a Migration

Once you have created your migration using one of the generators it’s time to
get to work!

3.1 Creating a Table

Migration method create_table will be one of your workhorses. A typical use
would be

create_table :products do |t|
t.string :name
end

which creates a products table with a column called name (and as discussed
below, an implicit id column).

The object yielded to the block allows you to create columns on the table. There
are two ways of doing it. The first (traditional) form looks like

The second form, the so called “sexy” migration, drops the somewhat redundant
column method. Instead, the string, integer, etc. methods create a column
of that type. Subsequent parameters are the same.

create_table :products do |t|
t.string :name, :null => false
end

By default, create_table will create a primary key called id. You can change
the name of the primary key with the :primary_key option (don’t forget to
update the corresponding model) or, if you don’t want a primary key at all (for
example for a HABTM join table), you can pass the option :id => false. If you
need to pass database specific options you can place an SQL fragment in the
:options option. For example,

removes the description and name columns, creates a part_number string
column and adds an index on it. Finally it renames the upccode column.

3.3 Special Helpers

Active Record provides some shortcuts for common functionality. It is for
example very common to add both the created_at and updated_at columns and so
there is a method that does exactly that:

create_table :products do |t|
t.timestamps
end

will create a new products table with those two columns (plus the id column)
whereas

change_table :products do |t|
t.timestamps
end

adds those columns to an existing table.

Another helper is called references (also available as belongs_to). In its
simplest form it just adds some readability.

create_table :products do |t|
t.references :category
end

will create a category_id column of the appropriate type. Note that you pass
the model name, not the column name. Active Record adds the _id for you. If
you have polymorphic belongs_to associations then references will add both
of the columns required:

3.4 Using the change Method

The change method removes the need to write both up and down methods in
those cases that Rails know how to revert the changes automatically. Currently,
the change method supports only these migration definitions:

add_column

add_index

add_timestamps

create_table

remove_timestamps

rename_column

rename_index

rename_table

If you’re going to need to use any other methods, you’ll have to write the
up and down methods instead of using the change method.

3.5 Using the up/down Methods

The down method of your migration should revert the transformations done by
the up method. In other words, the database schema should be unchanged if you
do an up followed by a down. For example, if you create a table in the up
method, you should drop it in the down method. It is wise to reverse the
transformations in precisely the reverse order they were made in the up
method. For example,

Sometimes your migration will do something which is just plain irreversible; for
example, it might destroy some data. In such cases, you can raise
ActiveRecord::IrreversibleMigration from your down method. If someone tries
to revert your migration, an error message will be displayed saying that it
can’t be done.

4 Running Migrations

Rails provides a set of rake tasks to work with migrations which boil down to
running certain sets of migrations.

The very first migration related rake task you will use will probably be
rake db:migrate. In its most basic form it just runs the up or change
method for all the migrations that have not yet been run. If there are
no such migrations, it exits. It will run these migrations in order based
on the date of the migration.

Note that running the db:migrate also invokes the db:schema:dump task, which
will update your db/schema.rb file to match the structure of your database.

If you specify a target version, Active Record will run the required migrations
(up, down or change) until it has reached the specified version. The version
is the numerical prefix on the migration’s filename. For example, to migrate
to version 20080906120000 run

$ rake db:migrate VERSION=20080906120000

If version 20080906120000 is greater than the current version (i.e., it is
migrating upwards), this will run the up method on all migrations up to and
including 20080906120000, and will not execute any later migrations. If
migrating downwards, this will run the down method on all the migrations
down to, but not including, 20080906120000.

4.1 Rolling Back

A common task is to rollback the last migration, for example if you made a
mistake in it and wish to correct it. Rather than tracking down the version
number associated with the previous migration you can run

$ rake db:rollback

This will run the down method from the latest migration. If you need to undo
several migrations you can provide a STEP parameter:

$ rake db:rollback STEP=3

will run the down method from the last 3 migrations.

The db:migrate:redo task is a shortcut for doing a rollback and then migrating
back up again. As with the db:rollback task, you can use the STEP parameter
if you need to go more than one version back, for example

$ rake db:migrate:redo STEP=3

Neither of these Rake tasks do anything you could not do with db:migrate. They
are simply more convenient, since you do not need to explicitly specify the
version to migrate to.

4.2 Resetting the database

The rake db:reset task will drop the database, recreate it and load the
current schema into it.

This is not the same as running all the migrations – see the section on
schema.rb.

4.3 Running specific migrations

If you need to run a specific migration up or down, the db:migrate:up and
db:migrate:down tasks will do that. Just specify the appropriate version and
the corresponding migration will have its up or down method invoked, for
example,

$ rake db:migrate:up VERSION=20080906120000

will run the up method from the 20080906120000 migration. These tasks still
check whether the migration has already run, so for example db:migrate:up
VERSION=20080906120000 will do nothing if Active Record believes that
20080906120000 has already been run.

4.4 Changing the output of running migrations

By default migrations tell you exactly what they’re doing and how long it took.
A migration creating a table and adding an index might produce output like this

If you want Active Record to not output anything, then running rake db:migrate
VERBOSE=false will suppress all output.

5 Using Models in Your Migrations

When creating or updating data in a migration it is often tempting to use one of
your models. After all, they exist to provide easy access to the underlying
data. This can be done, but some caution should be observed.

For example, problems occur when the model uses database columns which are (1)
not currently in the database and (2) will be created by this or a subsequent
migration.

Consider this example, where Alice and Bob are working on the same code base
which contains a Product model:

Bob goes on vacation.

Alice creates a migration for the products table which adds a new column and
initializes it. She also adds a validation to the Product model for the new
column.

6 Schema Dumping and You

6.1 What are Schema Files for?

Migrations, mighty as they may be, are not the authoritative source for your
database schema. That role falls to either db/schema.rb or an SQL file which
Active Record generates by examining the database. They are not designed to be
edited, they just represent the current state of the database.

There is no need (and it is error prone) to deploy a new instance of an app by
replaying the entire migration history. It is much simpler and faster to just
load into the database a description of the current schema.

For example, this is how the test database is created: the current development
database is dumped (either to db/schema.rb or db/structure.sql) and then
loaded into the test database.

Schema files are also useful if you want a quick look at what attributes an
Active Record object has. This information is not in the model’s code and is
frequently spread across several migrations, but the information is nicely
summed up in the schema file. The
annotate_models gem automatically
adds and updates comments at the top of each model summarizing the schema if
you desire that functionality.

6.2 Types of Schema Dumps

There are two ways to dump the schema. This is set in config/application.rb by
the config.active_record.schema_format setting, which may be either :sql or
:ruby.

If :ruby is selected then the schema is stored in db/schema.rb. If you look
at this file you’ll find that it looks an awful lot like one very big migration:

In many ways this is exactly what it is. This file is created by inspecting the
database and expressing its structure using create_table, add_index, and so
on. Because this is database-independent, it could be loaded into any database
that Active Record supports. This could be very useful if you were to distribute
an application that is able to run against multiple databases.

There is however a trade-off: db/schema.rb cannot express database specific
items such as foreign key constraints, triggers, or stored procedures. While in
a migration you can execute custom SQL statements, the schema dumper cannot
reconstitute those statements from the database. If you are using features like
this, then you should set the schema format to :sql.

Instead of using Active Record’s schema dumper, the database’s structure will be
dumped using a tool specific to the database (via the db:structure:dump Rake task)
into db/structure.sql. For example, for the PostgreSQL RDBMS, the
pg_dump utility is used. For MySQL, this file will contain the output of SHOWCREATETABLE for the various tables. Loading these schemas is simply a question
of executing the SQL statements they contain. By definition, this will create a
perfect copy of the database’s structure. Using the :sql schema format will,
however, prevent loading the schema into a RDBMS other than the one used to
create it.

6.3 Schema Dumps and Source Control

Because schema dumps are the authoritative source for your database schema, it
is strongly recommended that you check them into source control.

7 Active Record and Referential Integrity

The Active Record way claims that intelligence belongs in your models, not in
the database. As such, features such as triggers or foreign key constraints,
which push some of that intelligence back into the database, are not heavily
used.

Validations such as validates :foreign_key, :uniqueness => true are one way in
which models can enforce data integrity. The :dependent option on associations
allows models to automatically destroy child objects when the parent is
destroyed. Like anything which operates at the application level, these cannot
guarantee referential integrity and so some people augment them with foreign key
constraints in the database.

Although Active Record does not provide any tools for working directly with such
features, the execute method can be used to execute arbitrary SQL. You could
also use some plugin like foreigner
which add foreign key support to Active Record (including support for dumping
foreign keys in db/schema.rb).

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